A conventional split Stirling refrigeration system includes a reciprocating compressor and an expander cold finger. The piston of the compressor provides a nearly sinusoidal pressure variation in a pressurized refrigeration gas such as helium. The pressure variation in a head space is transmitted through a supply line to the expander.
Within the housing of the expander a cylindrical displacer is free to move in a reciprocating motion to change the volumes of a warm space and a cold space. The displacer contains a regenerative heat exchanger comprised of several hundred fine-mesh metal screen discs stacked to form a cylindrical matrix. Other regenerators, such as those with packed balls, are also known. Helium is free to flow through the regenerator between the warm space and the cold space. A piston element extends upwardly from the main body of the displacer into a gas spring volume at the warm end of the cold finger.
The refrigeration system can be seen as including two isolated volumes of pressurized gas. A working volume of gas comprises the gas in the space at the end of the compressor, the gas in the supply line, and the gas in the spaces and in the regenerator of the expander cold finger. The second volume of gas is the gas spring volume which is sealed from the working volume by a piston seal surrounding the drive piston. The displacer is driven at least partially by pressure differentials across the drive piston. Additional drive may be obtained by a linear drive motor in which the armature is coupled to the end of the piston.
Examples of prior Stirling cryogenic refrigerators can be found in U.S. Pat. Nos. 4,543,792 and 4,578,956.